Electric motor, particularly a brushless direct current motor

ABSTRACT

Electric motor with a substantially cylindrical air gap between the stator and the rotor, the stator being fitted to a bearing support for the rotor shaft bearing. In order to reduce noise emissions, the stator is connected to the bearing support by means of an elastic damper and the stator and bearing support are separated from one another by an air gap adjacent at least part of their facing faces.

This application is a continuation, of application Ser. No. 570,187filed Jan. 12, 1984, now abandoned.

The present invention relates to an electric motor, particularly in theform of a brushless direct current motor, wich is preferably intendedfor driving disk memories. However, it is also suitable for otherdriving functions, particularly in the office sphere. It has asubstantially cylindrical air gap between the stator and the rotor, thestator being fitted to a bearing member for bearing the rotor shaft.

The admissible noise emission values for equipment used at workinglocations in offices are constantly being reduced and this moreparticularly applies to computers used in such locations. Such computersnowadays have a vey high computing and storage capacity and aregenerally equipped with disk memories, particularly rigid or hard diskstores, as well as fans. The only components which emit noise in such acase are the driving motors for the disk memories and fans.

In connection with a disk memory, consideration could be given to theidea of e.g. either suspending the complete motor in an elastic manner,or to elastically fit the ball bearings used for rotor shaft bearingpurposes and, which as is known, constitute the main noise source.However, these two measures are not practicable in the case of a diskmemory, because e.g. in the case of a rigid disk memory,non-reproducible position changes of the rotor shaft, i.e. thenon-reproducible eccentricity, must e.g. be less than 1 μm.

SUMMARY OF THE INVENTION

The problem of the invention is to provide an electric motor,particularly a commutatorless or brushless direct current motor,suitable for driving disk memories, but also other equipment and whichhas a particularly low noise emission level and which can also have avery small, non-reproducible eccentricity.

On the basis of an electric motor of the aforementioned type, accordingto the invention this problem is solved in that the stator is connectedto the bearing support part by elastic damping means. Such aconstruction effectively reduces bearing noise, because sound wavesproduced in the bearing arrangement, e.g. the ball bearings, are nolonger reflected against the stator and are consequently prevented frompassing backwards and forwards between bearing and stator. However, inthe stator, electromagnetically produced noise is prevented from beingreflected backwards and forwards between the stator and the bearing.Such electromagnetically produced noise can in particular occur due toaxial and/or radial electromagnetic disturbing forces between rotor andstator.

The elastic damper is preferably axially spaced from the rotor shaftbearing. It can appropriately have a soft casting compound between thestator and the bearing member and, advantageously, following hardening,forms an intermediate part positively connected to the stator and/orbearing support. This intermediate member is appropriately circular.Such a damping arrangement can be manufactured particularly easily andensures extensive damping between the bearing support and the stator.According to a modified embodiment, the elastic damping means can alsohave one or more prefabricated elastic components, particularly elasticO-rings. In order to securely mount the stator on the bearing supportpart.[.,.]. of the cross-section of the O-ring preferably engages in acorresponding groove on the facing faces of the stator and/or bearingsupport.

According to another inventive solution proposal for the aforementionedproblem, the stator and the bearing support are separated from oneanother by an air gap over a substantial portion of their facing faces.Such an air gap leads to a significant reduction to the electric motornoise level. However, it can advantageously be provided in combinationwith the aforementioned elastic damping means. The air gap preferablyseparates the stator and the bearing support in the area critical fornoise transmission purposes and which is adjacent to the rotor shaftbearing.

In the case of the combined use of the elastic damping means and theaforementioned air gap, the stator and bearing support part arepreferably interconnected substantially only in the area of the elasticdamping means, while being separated from one another elsewhere by theair gap.

According to another feature of the invention, the rotor shaft ismounted in two ball bearings, with in each case different numbers ofballs. This feature also reduces noise emission and can be providedalone or together with one or more of the previously discussed inventivefeatures.

Electric motors of the aforementioned type, particularly for drivingdisk memories are known, in which the bearing support is in turnconnected to a mounting flange extending substantially perpendicular tothe rotor shaft axis. In the case of such an electric motor, theaforementioned problem can be solved by providing the mounting flangewith a sound-absorbing layer. This feature can also be used alone, orcan be combined with one or more of the aformentioned measures.

If, as is known per se (Ser. No. 440,537) the mounting flange carries aprinted circuit board and/or a magnetic shield, according to a furtherdevelopment of the invention, the mounting flange is connected with theprinted circuit board and/or the magnetic shield to form asound-absorbing multilayer body. A soft casting compound and/or anelastic adhesive can appropriately be provided between the layers of themultilayer body. It has been found that a significant noise reduction iseven possible if, for reasons of positional accuracy of the rotor axisrelative to the mounting flange, the latter and the bearing support areconstructed in one piece.

In the case of an electric motor, in which the rotor is constructed asan external rotor with a substantially cup-shaped rotor casing (cf e.g.Ser. No. 440,537), noise emissions are considerably reduced by openingslocated in the bottom of the rotor casing. This feature can also beprovided alone or in combination with one or more of the previousfeatures.

Corresponding to a further feature of the invention, in the case of anelectric motor of the aforementioned type, axial and/or radialelectromagnetic disturbing forces are minimized to reduce noise betweenthe rotor and stator and once again this feature can be used alone or inconjunction with one or more of the aforementioned features. Themagnetic components of the rotor and stator can be arrangedsymmetrically to the another to minimize disturbing forces. However,this solution is not always practicable with constructional means.

Particularly when in the case of a brushless direct current motor forthe rotor position-dependent commutation of the currents in the motorwindings, one or more galvanomagnetic sensors, e.g. Hall generators orHall-IC's are used, which are located in the influence range of therotor magnetic field, a certain axial projection of the rotor magnets isrequired on the side facing the sensor or sensors, in order to ensure amagnetic flux density adequate for controlling the sensors. On theopposite side, the axial projection is appropriately much smaller, inorder to economize on expensive magnetic material and/or the axialoverall length. Thus, a permanent magnetic rotor is obtained, which isarranged asymmetrically with respect to the axial plane of symmetry ofthe stator iron. The different sizes of the axial projections leads toan axial force being exerted on the rotor, whose magnet attempts toadjust itself symmetrically to the stator iron. This force is generallyrotation position-dependent, e.g. because the air gap between the rotorand the stator does not have the same dimensions throughout. This canlead to the aforementioned electromagnetically produced noise. However,in a further development of the invention, this can be counteracted insuch an asymmetrical arrangement in that the stator carries an endplate, which cooperates with the rotor magnet for the axialsymmetrization of the magnetic field and preferably defines at leastpart of the air gap in the vicinity of the larger projection.

According to a modified embodiment of the invention, electromagneticallycaused noise can be reduced in that, in the vicinity of the larger axialprojection, the induction in the central part of the rotor magneticpoles is made at least zonally weaker than in the marginal areas of saidpoles adjacent to the pole clearances. This also ensures an axial forcesymmetrization, together with a reliable response or operation of therotation position sensors.

What is decisive is that the magnetic components of the rotor and statorare arranged symmetrically to one another, i.e. that a magneticsymmetrization takes place in such a way that the sum of the magneticaxial forces between the rotor and the stator are as small as possibleand preferably zero in the case of the finally fitted motor.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is described in greater detail hereinafter relative tonon-limitative embodiments and with reference to the attached drawings,wherein:

FIG. 1 a section through a driving motor for a rigid disk storeconstructed according to the invention.

FIG. 2 a partial development of the rotor magnet of the motor of FIG. 1.

FIG. 3 a larger scale partial section through the fixing flange of themotor according to FIG. 1 with a printed circuit board and a magneticfield.

FIG. 4 a modified embodiment similar to FIG. 1 and in section.

FIG. 5 a section through a disk store driving motor in accordance with afurther modified embodiment of the invention.

FIG. 6 a plan view of the bottom of the rotor casing of the motoraccording to FIG. 5.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 shows an external rotor-type direct driving motor for rigid diskstores constructed as a brushless direct current motor, designatedoverall by reference numeral 10. The motor has a cup-shaped rotor casing11, which is concentric to a rotor shaft 12 and is fixed thereto bymeans of a bush 13, which is pressed into a central opening of the rotorcasing. The rotor casing 11, which is made from good magneticallyconducting material, has a plurality of permanent magnetic pieces or aone-part permanent magnetic ring 14 which, together with components 11to 13 form the rotor 15 of motor 10. The permanent magnetic ring 14 ispreferably made from a mixture of hard ferrite, e.g. barium ferrite, andelastic material and constitutes a so-called rubber magnet. The latteris trapezoidally or approximately trapezoidally radially magnetized overthe pole pitch in the case of a relatively small pole clearance. Rotorcasing 11 can be constructed as a deep-drawn part.

Included in stator 16 of motor 10 is in particular a winding core 17,which comprises the actual stator iron 18, generally in the form ofstator plates, as well as end plates 19, 20 and which carries a statorwinding 21. Winding core 17 is supported on a tubular bearing supportpart 22. Rotor shaft 12 is mounted in the bearing support 22 with theaid of two ball bearings 23, 24, whose facing faces are supported oncorresponding shoulders of bearing support part 22 and which havedifferent numbers of balls. A cup spring 25 engages with the bottom ofthe inner ring of ball bearings 23 and the face of bush 13 facing saidball bearing, so that the ball bearings are axially braced against oneanother. The bearing support means, together with a mounting flange 26,forms a one-piece die casting. Instead of this, the bearing support partcan also be located by force-fit in a hub connected to the mountingflange, or can be fixed to the latter in some other way, e.g. bysoldering. Magnetic ring 14 and winding core 17 define a substantiallycylindrical air gap 27.

As shown in FIG. 1, stator 16 is connected to bearing support 22 bymeans of an elastic damping arrangement, which is located axially aboveball bearings 23 and which comprises an annular intermediate member 28made from a soft casting compound. In the area located axially below theintermediate member 28 in FIG. 1, stator 16 and bearing support 22 areseparated from one another by a narrow air gap 29. During theinstallation of motor 10, initially a bead made from an elastic castingcompound, e.g. a polyurethane-hardener mixture, is placed in a groove 30on the circumferential surface of the bearing support 22. Stator 16 isthen placed on bearing support part 22, until the inner face of anannular shoulder 31 of end plate 20 engages with a portion 32 of thecircumferential surface of bearing support 22, while its end faceengages with a shoulder of the latter. Stator 16 is centered withrespect to the axis of rotor shaft 12. The casting compound is thenhardened. The elastic intermediate member 28 formed in this way ensuresa positive connection between stator 16 and bearing support 22, which,in conjunction with air gap 29, effectively damps the acoustic vibrationtransmission between components 16 and 22. The mechanical connection ofcomponents 16 to 22 is, according to the embodiment of FIG. 1, at apoint which is axially spaced from the location of the main noisesource. The wall of bearing support part 22 can elastically withdraw ormove aside in the vicinity of the bearing zone 34 of the outer ring ofball bearing 23. All this contributes to a considerable reduction ofnoise emissions.

A shielding plate 35 made from a good magnetically conducting materialand a printed circuit board 36 are located on the bottom of mountingflange 26. In conjunction with rotor casing 11 and ball bearing 24,shielding plate 35 prevents the escape of magnetic stray fields into thespace 37 taken up by the rigid storage disks. Drive electronics andpossibly a speed control circuit (not shown) are located on printedcircuit board 36. The e.g. aluminium die casting mounting flange 26 haslugs 38, which project through recesses in components 35, 36 and onwhich are mounted spring clips 39 for securing the shielding plate 35.As can be more closely seen in FIG. 3, between mounting flange 26 andshielding plate 35 is provided a casting compound, e.g. polyurethanelayer 40, while between the shielding plate 35 and printed circuit board36 is placed a layer 41 of epoxide resin adhesive or some othersound-absorbing material. Mounting flange 26, shielding plate 35 andprinted circuit board 36 are in this way combined into a sound-absorbingmultilayer body. It is obvious that the positions of the shielding plateand printed circuit board can be interchanged. In addition, both layers40, 41 can be made from casting compound, adhesive or some otheracoustic vibration-damping material.

Mounting flange 26 makes it possible to fit motor 10 to a partition ofthe rigid disk store which, in known manner (e.g. Ser. No. 127,404)separates space 37 from the remainder of the interior of the apparatus.A hub 42 for receiving one or more hard storage disks is fixed to theupper end of rotor shaft 12 in FIG. 1. In order to seal the bearingsystem of rotor shaft 12 with respect to the storage disk receptionarea, a magnetic fluid seal 43 is placed in the bearing support 22between hub 42 and bearing 24. Seal 43 comprises two pole pieces 44, 45a permanent magnetic ring 46 located between the pole pieces and amagnetic field which is introduced into an annular clearance 47 betweenmagnetic ring 46 and rotor shaft 12. A radial impeller 48 is fixed tothe outside of the base of rotor casing 11 and draws in air in thecentral air and discharges it radially outwards.

According to FIG. 1, a galvanomagnetic rotation position sensor, e.g. inthe form of a Hall generator 49 is provided for controlling thecommutation of motor 10. The sensor is soldered to the printed circuitboard 36 and is influenced by the field of annular magnet 14. To ensurethat sensor 49 reliably operates, the annular magnet 14 projects axiallyfurther over stator iron 18 on the side facing printed circuit board 36than on the side facing the bottom of rotor casing 11. In order tocounteract axial disturbing forces which are produced and which couldgive rise to noise, a stator end plate 50 is provided on the side withthe larger axial projection. End plate 50 projects into the area of theaxially further projecting annular magnet 14 and limits the air gap 27there in a predetermined partial zone. In this way, there is asymmetrization of the magnetic field. In addition to or instead of this,magnet 14 can be deliberately partially demagnetized in the vicinity ofthe large axial projection, as shown in FIG. 2. The annular magneticpoles are 51, the pole clearances 52 and the areas with reducedmagnetization 53. The partial demagnetization areas 53 are spaced fromthe pole clearances. This ensures a completely satisfactory operation ofsensor 49, while the partial demagnetization with respect to themagnetic axial forces acts in the same way as a shortening of the largerprojection. The present application hereby incorporates by reference theentire description and drawings of the commonly owned application Ser.No. 06/391,145, filed June 23rd, 1982, now U.S. Pat. No. 4,574,211.

The embodiment according to FIG. 4 coincides with that of FIG. 1 withthe exception that an O-ring 54 is provided in place of intermediatemember 28. O-ring 54 engages in an annular groove 55 on thecircumferential surface of bearing support 22. Optionally, acorresponding groove can be provided in the stator iron.

FIG. 5 shows a particularly flat motor 60, parts having the samefunction as those of the embodiment of FIG. 1 are given the samereference numerals and will not be explained again. In this case, thenoise reduction function is served by the sound-absorbing multilayerconstruction of mounting flange 26, shielding plate 35, printed circuitboard 36 and the interposed layers 40, 41, as well as openings 61distributed over the bottom of rotor casing 11. Rotor casing 11 providedwith the opening 61 constitutes a reversal of the principle of thepiston loudspeaker in a closed box. There are preferably sevenequidistant openings.

Of course, the aforementioned sound-absorption measures can in each casebe used alone or in random combinations. Sliding bearings can be used inplace of ball bearings.

We claim:
 1. An electric motor, particularly a brushless direct currentmotor, comprising:a stator and a rotor which define a substantiallycylindrical air gap therebetween, said stator including stator ironmeans; a rotor shaft supporting said rotor and mounted for commonrotation therewith; bearing means rotatingly mounting said rotor shaft;stationary, substantially tubular bearing support means disposed incoaxial relationship to said rotor shaft and having an outer facingperipheral wall, said bearing means being mounted inside of said bearingsupport means and being supported thereby, said stator having an innerfacing wall and being mounted on the outside of said bearing supportmeans and being supported thereby, the inner facing wall of the statorfacing the outer facing wall of the bearing support means; said statorwall and said bearing support means wall being separated from oneanother over at least a substantial portion of their facing faces by asecond air gap defined therebetween which extends also over asubstantial portion of the axial length of said stator iron means; andelastic damping means connecting said stator to said bearing supportmeans.
 2. An electric motor according to claim 1, wherein the elasticdamper means is disposed near the rotor shaft bearing and is preferablymade axially short.
 3. An electric motor according to claim 1, whereinthe elastic damping means includes a soft casting compound between thestator and the bearing support means.
 4. An electric motor according toclaim 3, wherein the soft casting compound, after hardening, forms anintermediate member positively connected to at least one of the statorand the bearing support means.
 5. An electric motor according to claim4, wherein the intermediate member has an annular shape.
 6. An electricmotor according to claim 1, wherein the elastic damping means is atleast one elastic O-ring.
 7. An electric motor according to claim 6,wherein the O-ring engages with part of its cross-section in acorresponding groove on the facing faces of at least one of the statorand bearing support means.
 8. An electric motor according to claim 1wherein the bearing support means is in turn connected to a mountingflange extending substantially perpendicularly to the rotor shaft axis,and wherein the mounting flange includes a sound-absorbing layer.
 9. Anelectric motor according to claim 8 wherein the rotor shaft is mountedin first and second ball bearings with a different number of ballsdisposed circumferentially in the first bearing as compared to thenumber of balls disposed circumferentially in the second bearing.
 10. Anelectric motor, particularly a brushless direct current motor,comprisinga stator and a rotor which define a substantially cylindricalair gap therebetween, said stator including stator iron means; a rotorshaft supporting said rotor and mounted for common rotation therewith;bearing means rotatingly mounting said rotor shaft; stationary,substantially tubular bearing support means disposed in coaxialrelationship to said rotor shaft and having an outer facing peripheralwall, said bearing means being mounted inside of said bearing supportmeans and being supported thereby, said stator having an inner facingwall and being mounted on the outside of said bearing support means andbeing supported thereby, the inner facing wall of the stator facing theouter facing wall of the bearing support means; said stator wall andsaid bearing support means wall being separated from one another over atleast a substantial portion of their facing faces by a second air gapdefined therebetween which extends also over a substantial portion ofthe axial length of said stator iron means.
 11. An electric motoraccording to claim 10, wherein the second air gap separates said statorfrom said bearing support means adjacent at least a portion of saidbearing means.
 12. An electric motor according to claim 10, wherein thebearing support means is in turn connected to a mounting flangeextending substantially perpendicularly to the rotor shaft axis, andfurther comprising a sound-absorbing layer on said mounting flange. 13.An electric motor according to claim 12, wherein the mounting flange hasmounted thereon at least one of a printed circuit board and a magneticshield, and wherein said sound-absorbing layer includes at least one ofsaid printed circuit board and magnetic shield to form a sound-absorbingmultilayer body.
 14. An electric motor according to claim 13, wherein atleast one of a soft casting compound and an elastic adhesive is providedbetween the layers of the multilayer body.
 15. An electric motoraccording to claim 12, wherein the mounting flange and the bearingsupport means are constructed in one piece.
 16. An electric motoraccording to claim 10, wherein electromagnetic forces act between therotor and stator and wherein the magnetic components of the rotor andstator are arranged symmetrically to one another to minimize themagnetic forces for noise reduction purposes.
 17. An electric motoraccording to claim 10, with a permanent magnetic rotor arrangedasymmetrically with respect to the axial plane of symmetry of the statoriron, further comprising an end plate mounted on the stator, which endplate together with the rotor magnet causes axial symmetrization of themagnetic field.
 18. An electric motor according to claim 17, wherein therotor magnet projects axially over the stator iron at its two end facesand the end plate defines at least part of the air gap adjacent thelarger projection of the rotor.
 19. An electric motor according to claim10, with a permanent magnetic rotor having clearances between adjacentpoles, wherein the rotor magnet projects axially over the stator iron atits two end faces and adjacent the larger projection the induction inthe central area of the rotor magnetic poles is at least zonally weakerthan in the marginal regions of the rotor magnetic poles adjacent thepole clearances.
 20. An electric motor according to claim 10 wherein therotor shaft is mounted in first and second ball bearings with a diferentnumber of balls disposed circumferentially in the first bearing ascompared to the number of balls disposed circumferentially in the secondbearing.
 21. An electric motor, particularly a brushless direct currentmotor, comprisinga stator and .Iadd.a .Iaddend.rotor which define asubstantially cylindrical air gap therebetween, said stator includingstator iron means; a rotor shaft supporting said rotor and mounted forcommon rotation therewith; bearing means rotatingly mounting said rotorshaft; stationary, substantially tubular bearing support means disposedin coaxial relationship to said rotor shaft, said bearing means beingmounted inside of said bearing support means and being supportedthereby, said stator being mounted on the outside of said bearingsupport means and being supported thereby; said bearing means includingfirst and second ball bearings with a different number of balls disposedcircumferentially in the first bearing as compared to the number ofballs disposed circumferentially in the second bearing.
 22. An electricmotor, particularly a brushless direct current motor, comprisinga statorand a rotor which define a substantially cylindrical air gaptherebetween, said stator including stator iron means; a rotor shaftsupporting said rotor and mounted for common rotation therewith; bearingmeans rotatingly mounting said rotor shaft; stationary, substantiallytubular bearing support means disposed in coaxial relationship to saidrotor shaft and having an outer facing peripheral wall, said bearingmeans being mounted inside of said bearing support means and beingsupported thereby, said stator having an inner facing wall and beingmounted on the outside of said bearing support means and being supportedthereby, the inner facing wall of the stator facing the outer facingwall of the bearing support means; said stator wall and said bearingsupport means wall being separated from one another over at least asubstantial portion of their facing faces by a second air gap definedtherebetween which extends also over a substantial portion of the axiallength of said stator iron means; said rotor being constructed as anexternal rotor with a substantially cup-shaped rotor casing havingopenings distributed over the closed end of the cup-shaped casing. 23.An electric motor, particularly a brushless direct current motor,comprisinga stator and a rotor which define a substantially cylindricalair gap therebetween, said stator including stator iron means; a rotorshaft supporting said rotor and mounted for common rotation therewith;bearing means rotatingly mounting said rotor shaft; stationary,substantially tubular bearing support means disposed in coaxialrelationship to said rotor shaft and having an outer facing peripheralwall, said bearing means being mounted inside of said bearing supportmeans and being supported thereby, said stator having an inner facingwall and being mounted on the outside of said bearing support means andbeing supported thereby, the inner facing wall of the stator facing theouter facing wall of the bearing support means; said stator wall andsaid bearing support means wall being separated from one another over atleast a substantial portion of their facing faces by a second air gapdefined therebetween which extends also over a substantial portion ofthe axial length of said stator iron means; elastic damping meansconnecting said stator to said bearing support means; said rotor beingconstructed as an external rotor with a substantially cup-shaped rotorcasing having openings distributed over the closed end of the cup-shapedcasing.
 24. An electric motor, particularly a brushless direct currentmotor, comprisinga stator and a rotor which define a substantiallycylindrical air gap therebetween, said stator including stator ironmeans and a coil creating an electromagnetic field when energized, saidrotor mounting a permanent magnet ring, electromagnetic forces actingbetween the rotor and the stator when the coil is energized; a rotorshaft supporting said rotor and mounted for common rotation therewith;bearing means rotatingly mounted said rotor shaft; stationary,substantially tubular bearing support means disposed in coaxialrelationship to said rotor shaft, said bearing means being mountedinside of said bearing support means and being supported thereby, saidstator being mounted on the outside of said bearing support means andbeing supported thereby; the magnetic components of the rotor and statorbeing arranged symmetrically to one another whereby the sum of themagnetic axial forces between the rotor and stator are minimized fornoise reduction purposes. .Iadd.
 25. An electric motor according toclaim 24, with a permanent magnetic rotor arranged asymmetrically withrespect to the axial plane of symmetry of the stator iron, furthercomprising an end plate mounted on the stator, which end plate togetherwith the rotor magnet causes axial symmetrization of the magnetic field..Iaddend. .Iadd.26. An electric motor according to claim 25, wherein therotor magnet projects axially over the stator iron at its two end facesand the end plate defines at least part of the air gap adjacent thelarger projection of the rotor. .Iaddend. .Iadd.27. An electric motoraccording to claim 24, with a permanent magnetic rotor having clearancesbetween adjacent poles, wherein the rotor magnet projects axially overthe stator iron at its two end faces and adjacent the larger projectionthe induction in the central area of the rotor magnetic poles is atleast zonally weaker than in the marginal regions of the rotor magneticpoles adjacent the pole clearances. .Iaddend. .Iadd. A brushless directcurrent outer rotor motor for a hard disk drive, comprisinga stator anda rotor which define a substantially cylindrical air gap therebetween,said stator including stator iron means and a coil creating anelectromagnetic field when energized, said rotor mounting a permanentmagnet ring, electromagnetic forces acting between the rotor and thestator when the coil is energized; a rotor shaft supporting said rotorand mounted for common rotation therewith; bearing means rotatinglymounting said rotor shaft; stationary, substantially tubular bearingsupport means disposed in coaxial relationship to said rotor shaft, saidbearing means being mounted inside of said bearing support means andbeing supported thereby, said stator being mounted on the outside ofsaid bearing support means and being supported thereby; the magneticcomponents of the rotor and stator being arranged with respect to oneanother to reduce axial magnetic forces for noise reduction purposes..Iaddend. .Iadd.29. An electric motor according to claim 28, wherein themagnetic components of the rotor and stator are arranged symmetricallyto one another to minimize the magnetic forces for noise reductionpurposes. .Iaddend. .Iadd.30. An electric motor according to claim 28,wherein the arrangement of rotor and stator magnetic components fornoise reduction purposes includes the permanent magnetic rotor beingarranged asymmetrically with respect to the axial plane of symmetry ofthe stator iron, further comprising an end plate mounted on the stator,which end plate together with the rotor magnet causes axialsymmetrization of the magnetic field. .Iaddend. .Iadd.31. An electricmotor according to claim 30, wherein the rotor magnet projects axiallyover the stator iron at its two end faces and the end plate defines atleast part of the air gap adjacent the larger projection of the rotor..Iaddend. .Iadd.32. An electric motor according to claim 28, wherein thearrangement of rotor and stator magnetic components for noise reductionpurposes includes the permanent magnetic rotor having clearances betweenadjacent poles, wherein the rotor magnet projects axially over thestator iron at its two end faces and adjacent the larger projection theinduction in the central area of the rotor magnetic poles is at leastzonally weaker than in the marginal regions of the rotor magnetic polesadjacent the pole clearances. .Iaddend. .Iadd.33. A brushless directcurrent outer rotor motor for a hard disk drive, comprising a stator anda rotor which define a substantially cylindrical air gap therebetween,said stator including stator iron means and a coil creating anelectromagnetic field when energized, said rotor mounting a permanentmagnet ring, electromagnetic forces acting between the rotor and thestator when the coil is energized; a rotor shaft supporting said rotorand mounted for common rotation therewith; bearing means rotatinglymounting said rotor shaft; stationary, substantially tubular bearingsupport means disposed in coaxial relationship to said rotor shaft, saidbearing means being mounted inside of said bearing support means andbeing supported thereby, said stator being mounted on the outside ofsaid bearing support means and being supported thereby; the magneticcomponents of the rotor and stator being arranged symmetrically to oneanother whereby the sum of the magnetic axial forces between the rotorand stator are minimized for noise reduction purposes. .Iaddend.